Energy storage is a factor intimately linked with the development of electrical systems within the framework of current and future cities. In this progressive society the concept of a Smart City is becoming firmly established as one of the fundamental elements in innovative city design and is a priority within the EU's Horizon 2020. The development of these new cities, where a vast number of devices and sensors gather and exchange signals from a range of sources, creates a requirement within the electrical storage and supply sector. As a result there is a need to develop intelligent networks capable of improving energy storage, with the aim of achieving greater flexibility and stability in the network and thus facilitating the quality and management of electrical supply.
Due to the high number of devices and sensors and the diverse location of these within cities, mains power supply can often be difficult or costly to install. As a result, primary or rechargeable batteries are frequently used to supply these devices. The high cost of the rechargeable batteries and the inconvenience of recharging favours the choice of primary batteries where the only maintenance required is their replacement when they run down. In this regard the low consumption of the devices and sensors permits a high degree of independence, in such a way that their replacement may be extended over long periods of time. The primary electrochemical accumulators currently on sale such as alkaline, zinc-air or primary lithium batteries etc. possess certain limitations in terms of energy requirements, weight/volume or price. As a result there is a need to develop batteries with improved specific energy capacities which would allow less frequent maintenance of the devices and improvement in terms of weight and volume requirements.
Within this investigative framework, metal-air batteries have demonstrated the potential for storing more energy than lithium-ion batteries, currently used in electrical vehicles and various applications on the electrical mains network. In a metal-air battery, the metal (such as zinc. aluminium. lithium, etc.), reacts with the oxygen in the air to generate electricity.
Depending on the materials used, metal-air batteries may also be less expensive than lead-acid batteries, which are currently the cheapest rechargeable batteries most frequently used in photovoltaic and automobile power applications.
Although non-rechargeable metal-air batteries have been used commercially for a long time (frequently used in hearing aids for example), they present the problem of being difficult to recharge electrically. In this regard, to repeatedly recharge a metal-air battery it is necessary to extract the oxygen and reform the metal element again. However, the structures the metal tends to form as a result present certain problems which make the reversibility of the oxidation-reduction reactions of the process difficult.
On the other hand, creating a long lasting air electrode (the point of interaction between the battery and the exterior) is also difficult. Existing examples work well for single use batteries, but not for rechargeable batteries intended for longer life.
In all events, the charging of an aqueous electrolyte is impossible for various reasons, making it necessary to replace the electrolytes with organic compounds or ionic liquids.
Nevertheless, there are numerous inventions related to this technology, based on lithium-air batteries, which have been shown to have four times more energy than the traditional lithium-ion, zinc-air or the recent aluminium-air.
In this way, for example, in WO2004082060 a high capacity air battery is described which comprises a structure for the storage of air designed to retain and transfer oxygen as the active material of the positive electrode and an electrolyte capable of absorbing the dampness in the air which comprises at least aluminium chloride and calcium chloride. As the active material of the negative electrode either aluminium or an aluminium alloy is used.
In US2010285375 a metal-air electrochemical cell is described in which a low temperature ionic liquid is used as the electrolyte.
In US2009053594 an air battery is described which comprises an air cathode with a porous carbon structure and which contains a non-aqueous organic solution electrolyte which comprises a lithium and alkylenecarbonate salt as an additive.
In WO2011061728 air batteries are described which comprise silicon as an anode and air as a cathode to separate the oxygen and a non-aqueous electrolyte.
Finally, in US2015093659 an electrochemical cell is described capable of generating and/or accumulating electrical energy which comprises an aluminium or aluminium alloy electrode and a non-aqueous electrolyte which contains a mixture of AlCl3 and a chlorinated cycle or heterocycle of an aliphatic nitrogen derivative.
Within these technologies, the present invention is included among aluminum-air batteries, with aluminium being a very attractive material in the field of energy storage. This is due to the fact that aluminium is a very accessible metal which, when compared with other systems from the same family, also presents a high gravimetric energy density (close to 3 Ah/g) comparable to that of lithium (3.86 Ah/g). Furthermore its volumetric energy density (8.04 Ah/cm3) is four times higher than that of lithium and it provides a voltage per cell similar to alkaline batteries using nickel electrodes.
Nevertheless one of the disadvantages of this technology, and the reason it is not currently commercially successful, is the self-corrosion of aluminium in alkaline electrolytes. As a product of this corrosion active material (Al) is lost and hydrogen is given off in a spontaneous manner. This results in an uncontrolled loss of energy, and a corresponding reduction in the active life of the battery subject to this corrosion.
The object of this invention is therefore to present a solution which inhibits the corrosion of batteries and electrochemical cells without sacrificing the other parameters of these such as the electric potential difference or the high specific energy.